Crystals, such as quartz crystal oscillator, are widely used in frequency sources of electronic circuits, such as jitter cleaner and frequency synthesizer. However, it is also well known that the natural resonant frequency of such crystals changes when the crystals are subjected to acceleration (cf. Reference [1]). Due to the acceleration sensitivity of crystal oscillator, mechanical vibration could induce additional phase noise to its output clock which can unfavorably cause performance degradation or even malfunction to numerous applications.
The vibration effect is obvious in three typical real-world scenarios: 1) a wireless base station on an iron tower shaken by wind, passing trucks or earthquakes; 2) vehicle-carried equipments, such as wireless communication terminals, radar, navigator and/or scientific instrument on cars, trains, ships, airplane, helicopter, rockets, or space shuttle; and 3) high-speed network/digital communication equipments, e.g. high-speed gateway applications, high speed routers, high-throughput cloud storage equipments. These equipments require a system clock with ultra low phase noise which may be degraded by vibrations from cooling fans or human operations (walking, closing doors).
Considering that the PLL is a fundamental module widely used in electronic devices, vibration-induced phase noise may degrade key performances of electronic systems, e. g. connectivity, signal/noise ratio and/or bit error rate of communication system, the resolution and/or reliability of radar and other sensors. Therefore, a technology to suppress the vibration-induced phase noise is very valuable to many markets such as cellular networks, smart-phones, 2-way radios, vehicle-to-vehicle communication, radar, aerospace, internet and/or cloud facilities, and etc.
The well-known methods for reducing the acceleration sensitivity of crystal oscillator include both active and passive methods.
The passive approach involves either a special mechanical structure to absorb vibration or mounting pairs of oppositely oriented crystals to cancel the acceleration-induced frequency shifts. Such methods have been applied in both print circuit board (PCB) level (cf. Reference [2]) or micro structure integrated in compact sized packages (cf. References [3]-[7]).
In the active method, mechanical vibration may be sensed by acceleration sensors and fed back to dynamically adjust an output frequency of the crystal oscillator through its tuning interface. There are usually three accelerometers closely placed to the crystal oscillator to sense the accelerations of 3 axes in free space.
In 1981 (cf. Reference [8]) and 1987 (cf. Reference [9]), V. R. Rosati et al. proposed a solution in which the tuning signal generated by the accelerometer may be modified by an analog amplifier with a fixed gain and no phase delay.
In 1989 (cf. Reference [10]), Frerking described using an adaptive filter in a polarization-effect tuning method to minimize a voltage controlled crystal oscillator's (VCXO) vibration-induced phase noise.
In 1997 (cf. Reference [11]), Wei proposed an adaptive signal conditioner comprising one or more accelerometers, a bandpass filter and an analog circuit representing transfer function H(s) to suppress the vibration-induced phase noise of the VCXO.
In 2006 (cf. Reference [20]), Nicola et al. proposed a device for stabilizing the PLL. In Nicola's patent, the stabilizing device may add a random digital signal to the oscillator control signal and cross-correlates an output of a phase detector and said random digital signal to estimate a transfer function and a loop gain of the PLL. The estimated transfer function and loop gain may be then used to adjust the gain of two amplifiers through a lookup table to stabilize the transfer function of PLL when the jitter of reference clock changes. In Nicola's patent, the variance and gain of the signal added into the oscillator control signal are pre-determined, and the loop filter is adjusted. It is only be applicable to all digital PLL and not aims to suppress the vibration induced phase noise.
Existing passive and active methods suffer certain drawbacks which either limit its performance or bring high product costs.
Therefore, a technical solution of adaptively compensating vibration in the PLL is desired.